As demands for good performance, low noise and low cost RF components in modem communications increase, conventional Si-based devices can no longer meet the new requirements of performance specifications, output power and linearity, and thus have been replaced by power SiGe HBTs (Silicon-Germanium Heterojunction Bipolar Transistors) which are playing an important role in power amplifiers having a wider band and higher frequency.
Although the frequency performance of SiGe HBTs is not as good as GaAs-based devices, SiGe HBTs have well solved the heat dissipation problem of power amplifiers due to their better thermal conductivities and substrate mechanical properties. Additionally, SiGe HBTs have better linearity and are better integrated. Since SiGe HBTs belong to Si-based technology, they have good compatibility with CMOS (Complementary Metal Oxide Semiconductor) processes. SiGe BiCMOS process has provided a great convenience for the integration of power amplifiers and logic control circuits and has also reduced process costs.
At present, SiGe HBTs have been widely used as high-frequency, high-power power amplifier devices in wireless communication products, such as power amplifiers and low noise amplifiers in mobile phones. The output power of an RF power amplifier can be effectively improved by increasing its operating current or operating voltage within its normal operating range. In addition, for a circuit employing a SiGe HBT, improving the voltage withstanding capability of the SiGe HBT would allow the circuit to obtain a lower current under the same power, so as to reduce the power consumption. Therefore, it has been a most important technical problem to reduce the base region resistance of a SiGe HBT so as to reduce the power consumption and increase the maximum oscillation frequency of the device.
However, in the prior art, the SiGe epitaxial layer in the base region of a SiGe HBT has been made thinner and thinner, for example, less than 50 nm, to obtain a higher cut-off frequency of the device. A thin base region will lead to an increase in the base region resistance, which is against the improvement of other performances of the device.